Improved carbon fiber reinforced composite article

ABSTRACT

A process is provided for modifying the surface characteristics of a carbonaceous fibrous material (i.e., either amorphous carbon or graphitic carbon) and to thereby facilitate enhanced adhesion between the carbonaceous fibrous material and a matrix material. The carbonaceous fibrous material is coated with a compact polyphenylene polymer coating which is deposited thereon upon contact with an excited gas species generated by applying high frequency electrical energy in pulsed form to an inert gas in the presence of at least one aromatic compound. The coating step is efficiently conducted at a temperature of about 0* to 150*C. and at a pressure within the coating zone of about 1 to 3 atmospheres. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

Ilnited tates atertt [191 IMPROVED CARBON FIBER REINFORCED COMPOSITEARTICLE.

[75] Inventor: Kenneth C. Hou, Whippany, NJ.

[73] Assignee: Celanese Corporation, New York,

[22] Filed: Feb. 28, 1973 [21] Appl. No.: 336,868

Related U.S. Application Data [62] Division of Ser. No. 142,656, May 12,1971, Pat. No.

[56] References Cited UNITED STATES PATENTS 3,002,850 10/1961 Fischer r117/33 3,108,018 10/1963 Lewis ll7/l6l 3,238,054 3/1966 Bickedike et a1.117/46 3,421,930 l/l969 Knox et a1. 1l7/93.1 3,429,739 2/1969 Tittmannet a1. 117/106 [111 Dec. 10, 11974 Rohl et a1. 117/46 PrimaryExaminerRalph S. Kendall Assistant Examiner-P. E. Willis 5 7 ABSTRACT Aprocess is provided for modifying the surface characteristics of acarbonaceous fibrous material (i.e., either amorphous carbon orgraphitic carbon) and to thereby facilitate enhanced adhesion betweenthe carbonaceous fibrous material and a matrix material. Thecarbonaceous fibrous material is coated with a compact polyphenylenepolymer coating which is deposited thereon upon contact with an excitedgas species generated by applying high frequency electrical energy inpulsed form to an inert gas in the presence of at least one aromaticcompound. The coating step is efficiently conducted at a temperature ofabout 0 to 150C. and at a pressure within the coating zone of about 1 to3 atmospheres. Composite articles of enhanced interlaminar shearstrength may be formed by incorporating the fibers modified inaccordance with the present process in a resinous matrix material.

9 Claims, 8 Drawing Figures GENERATOR PATENIEB 308531.600

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SOUQCE PULSE GENERATOR PATENTEB BEE? 0 I974 SHEEF 2 [IF A /Z m M fi -llM Q F a U F W P L/ 7 7 C v 4 a K c K f a W TUNE h WM M BACKGROUND OF THEINVENTION In the search for high performance materials, considerableinterest has been focused upon carbon fibers. The term carbon fibers" isused herein in its generic sense and includes graphite fibers as well asamorphous carbon fibers. Graphite fibers are defined herein as fiberswhich consist essentially of carbon and have a predominant x-raydiffraction pattern characteristic of graphite. Amorphous carbon fibers,on the other hand, are defined as fibers in which the bulk of the fiberweight can be attributed to carbon and which exhibit an essentiallyamorphous x-ray diffraction pattern. Graphite fibers generally have ahigher Youngs modulus than do amorphous carbon fibers and in additionare more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, and carbonfibers theoretically have among the best properties of any fiber for useas high strength reinforcement. Among these desirable properties arecorrosion and high temperature resistance, low density, high tensilestrength, and high modulus. Graphite is one of the very few knownmaterials whose tensile strength increases with temperature. Uses forcarbon fiber reinforced composites include aerospace structuralcomponents, rocket motor casings, deep-submergence vessels, and ablativematerials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use aspossible matrices in which carbon fibers may be incorporated to providereinforcement and produce a composite article. The matrix material whichis utilized is commonly a thermosetting resinous material and iscommonly selected because of its ability to also withstand highlyelevated temperatures.

While it has been possible in the past to provide carbon fibers ofhighly desirable strength and modulus characteristics, difficulties havearisen when one attempts to gain the full advantage of such propertiesin the resulting carbon fiber reinforced composite article. Suchinability to capitalize upon the superior single filament properties ofthe reinforcing fiber has been traced to inadequate between the fiberand the matrix in the resulting composite article.

Various techniques have been proposed in the past for modifying thefiber properties of a previously formed carbon fiber in order to makepossible improved adhesion when present in a composite article. See, forinstnce. British Pat. No. 1,180,441 to Nicholas J. Wadsworth and WilliamWatt wherein it is taught to' heat a carbon fiber normally within therange of 350C. to 850C. (e.g., 500 to 600C.) in an oxidizing atmospheresuch as air for an appreciable period of time (e.g., 1 hour or more).

It has also been proposed in the past that coupling agents be depositedupon carbon fibers via an electric discharge plasma producing reactionwhich is conducted at a pressure substantially below atmospheric (e.g.,2 mm. Hg). Such techniques have proven to be tedious and not readilyadaptable to commercial utilization because of the necessity ofoperating at a substantially reduced pressure.

It is an object of the invention to provide a process for efficientlymodifying the surface characteristics of carbon fibers with nosubstantial reduction in the single filament tensile properties.

It is an object of the invention to provide a process for improving theability of carbon fibers to bond to a resinuous matrix material.

It is an object of the invention to provide a process for modifying thesurface characteristics of carbon fibers which may be conductedrelatively rapidly at moderate temperatures at atmospheric pressure.

It is another object of the invention to provide composite articlesreinforced with carbon fibers exhibiting improved interlaminar shearstrength.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved process forthe modification of the surface characteristics of a carbonaceousfibrous material containing at least about percent carbon by weightcomprises: (a) providing in a coating zone at a pressure of about 1 to 3atmospheres an inert gas and at least one aromatic compound having from1 to 4 six member carbon rings which is capable of undergoingpolymerization to form a polyphenylene polymer, (b) applying highfrequency electrical power in pulsed from to the inert gas sufficient toestablish an excited gas species within the coating zone whilemaintaining the temperature of the zone at about 0 to C, and (c)contacting the carbonaceous fibrous material while present in thecoating zone with the excited gas species until a. compact coating of apolyphenylene polymer is deposited on the carbonaceous fibrous materialhaving a thickness of about 25 to 800 angstrom units.

The resulting carbon fibers 'may be incorporated in a resinuous matrixmaterial to form a composite article exhibiting enhanced interlaminarshear strength.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration of arepresentative apparatus arrangement for modifying the surfacecharacteristics of a carbonaceous fibrous material in accordance withthe present invention.

FIGS. IA and IB are schematic illustrations of alternative means forcapacitively exciting the inert gas in the coating zone of FIG. 1.

FIG. IC is a schematic illustration of means for inductively excitingthe inert gas in the coating zone of FIG. I.

FIG. 2 is a schematic illustration of a further representative apparatusarrangement for modifying the surface characteristics of a carbonaceousfibrous material in accordance with the present process wherein amercury pool surrounds the surface modification zone.

FIG. 3 is a photograph made with the aid of a scan ning electronmicroscope of a portion of a graphic filament which has not undergonesurface modification. (5500X) FIG. 41 is a photograph made with the aidof a scanning electron microscope of a portion of graphite filamentbearing a compact coating of a polyphenylene polymer which isrepresentative of the appearance of carbon fibers which are surfacemodified in accordance with the present process. (4900X) FIG. 5 is aphotograph made with the aid of a scanning electron microscope of aportion of a graphite filament bearing a non-compact coating of apolyphenylene polymer which is representative of the appearance ofcarbon fibers which are surface modified while providing the coatingzone at an excessive temperature. (SSOOX) DESCRIPTION OF PREFERREDEMBODIMENTS The Starting Material The fibers which are surface modifiedin accordance with the present process are carbonaceous and contain atleast about 90 per cent carbon by weight. Such carbon fibers may exhibiteither an amorphous carbon or a predominantly graphitic carbon x-raydiffraction pattern. In a preferred embodiment of the process thecarbonaceous fiber which undergo surface coating contain at least about95 per cent carbon by weight, and at least about 99 per cent carbon byweight in a particularly preferred embodiment of the process.

The carbonaceous fibrous material may be provided as either a continuousor a discontinuous length. In a preferred embodiment of the process thecarbonaceous fibrous material is a continuous length which may be anyone of a variety of physical configuration provided substantial accessto the fiber surface is possible during the surface coatingtreatment-described hereafter. For instance, the carbonaceous fibrousmaterial may assume the configuration of a continuous length of amultifilament yarn, tow, tape, strand, cable, or similar fibrousassemblage. In a preferred embodiment of the process the carbonaceousfibrous material is one or more continuous multifilament yarn. When aplurality of multifilament yarns are surface coated simultaneously, theymay be continuously passed through the heating zone while in paralleland in the form of a flat ribbon.

The carbonaceous fibrous material which is treated in the presentprocess optionally may be provided with a twist which tends to improvethe handling characteristics. For instance, a twist of about 0.1 to 5tpi, and preferably about 0.3 to 1.0 tpi, may be imparted to amultifilament yarn. Also, a false twist may be used instead of or inaddition to a real twist. Alternatively, one may select continuousbundles of fibrous material a which possess essentially no twist.

. mally stabilized material is heated to a maximum temperature of 2,000to 3,100C. (preferably 2,400 to 3,100C.) in an inert atmosphere,substantial amounts of graphitic carbon are commonly detected in theresulting carbon fiber, otherwise the carbon fiber will commonly exhibitan essentially amorphous x-ray diffraction pattern.

The exact temperature and atmosphere utilized during the initialstabilization of an organic polymeric fibrous material commonly varywith the composition of the precursor as will be apparent to thoseskilled in the art. During the carbonization reaction elements presentin the fibrous material other than carbon (e.g., oxygen and hydrogen)are substantially expelled. Suitable organic polymeric fibrous materialsfrom which the fibrous material capable of undergoing carbonization maybe derived include an acrylic polymer, a cellulosic polymer, apolyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussedhereafter, acrylic polymeric materials are particularly suited for useas precursors in the formation of carbonaceous fibrous materials.Illustrative examples of suitable cellulosic materials include thenatural and regenerated forms of cellulose, e.g., rayon. Illustrativeexamples of suitable polyamide materials include the aromaticpolyamides, such as nylon 6T, which is formed by the condensation ofhexamethylenediarnine and terephthalic acid. An illustrative example ofa suitable polybenzimidazole ispoly-2,2-mphenylene-5,5'-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about mol per cent ofrecurring acrylonitrile units with not more than about 15 mol per centof a monovinyl compound which is copolymerizable with acrylonitrile suchas styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred carbonaceous fibers material for usein the present process multifilament bundles of an acrylic fibrousmaterial may be initially stabilized in an oxygen-containing atmosphere(i.e., preoxidized) on a continuous basis in accordance with theteachings of United States Ser. No. 749,957, filed Aug. 5, 1968 (nowabandoned), of Dagobert E. Stuetz, which is assigned to the sameassignee as the present invention and is herein incorporated byreference. More specifically, the acrylic fibrous material should beeither an acrylonitrile homopolymer or an acrylonitrile copolymer whichcontains no more than about 5 mol per cent of one or more monovinylcomonomers copolymerized with acrylonitrile. In a particularly preferredembodiment of the process the fibrous material is derived from anacrylonitrile homopolymer. The stabilized acrylic fibrous material whichis preoxidized in an oxygen-containing atmosphere is black inappearance, commonly contains a bound oxygen content of at least about 7per cent by weight as determined by the Unterzaucher analysis, retainsits original fibrous configuration essentially intact, and is nonbumingwhen subjected to an ordinary match flame.

In preferred techniques for forming the starting material for thepresent process a stabilized acrylic fibrous material is carbonized andgraphitized while passing through a temperature gradient present in aheating zone in accordance with the procedures described in commonlyassigned United States Pat. Nos. 777,275, filed Nov. 20, 1968 of CharlesM. Clarke (now abandoned); 17,780, filed Mar. 9, 1970 of Charles M.Clarke, Michael J. Ram, and John P. Riggs (now U.S. Pat. No. 3,677,705);and 17,832, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, andArnold J. Rosenthal (now US. Pat. No. 3,775,520). Each of thesedisclosures is herein incorporated by reference.

In accordance with a particularly preferred carbonization andgraphitization technique a continuous length of stabilized acrylicfibrous material which is non-burning when subjected to an ordinarymatch flame and derived from an acrylic fibrous material selected fromthe group consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers which contain at least about 85 mol per cent of acrylonitrileunits and up to about mole per cent of one or more monovinyl unitscopolymerized therewith is converted to a graphitic fibrous materialwhile preserving the original fibrous configuration essentially intactwhile passing through a carbonization/graphitization heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich the fibrous material is raised within a period of about to about300 seconds from about 800C. to a temperature of about 1,600C. to form acontinuous length of carbonized fibrous material, and in which thecarbonized fibrous material is subsequently raised from about 1,600C. toa maximum temperature of at least about 2,400C. within a period of about3 to 300 seconds where it is maintained for about 10 seconds to about200 seconds to form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to produce thecarbonaceous starting material may be varied as will be apparent tothose skilled in the art. It is essential that the apparatus selected becapable of producing the required temperature while excluding thepresence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous materialundergoing carbonization is heated by use of an induction furnace. Insuch a procedure the fibrous material may be passed in the direction ofits length through a hollow graphite tube or other susceptor which issituated Within the windings of an induction coil. By varying the lengthof the graphite tube, the length of the induction coil, and the rate atwhich the fibrous material is passed through the graphite tube, manyapparatus arrangements capable of producing carbonization orcarbonization and graphitization may be selected. For large scaleproduction, it is of course preferred that relatively long tubes orsusceptors be used so that the fibrous material may be passed throughthe same at a more rapid rate while being carbonized or carbonized andgraphitized. The temperature gradient of a given apparatus may bedetermined by conventional optical pyrometer measurements as will beapparent to those skilled in the art. The fibrous material because ofits small mass and relatively large surface area instantaneously assumesessentially the same temperature as that of the zone through which it iscontinuously passed.

The Contents of the Coating Zone Within the coating zone is provided ininert gas at a pressure of about I to 3 atmospheres as well as aromaticcompound capable of undergoing polymerization to form a polyphenylenepolymer. The necessity of operating at reduced pressure conditions andthe concomitant disadvantages associated therewith are accordinglyavoided in the present process. The coating zone is convenientlyprovided at substantially atmospheric pressure.

Suitable inert gases for inclusion in the coating zone include nitrogen,helium, argon, neon, krypton, and xenon, and mixtures of the foregoing.The preferred inert gases are monoatomic, e.g., helium, argon, neon,krypton, and xenon since these tend to undergo excitation more readily.The particlarly preferred monoatomic inert gases for use in the processare helium and argon. The relatively high current costs of neon,krypton, and xenon militate against their selection. When present in thecoating zone, the inert gas undergoes excitation upon application of thehigh frequency electrical power in pulsed form (described hereafter) andaids in the generation of an excited gas species capable of promotingthe formation of polyphenylene polymer. In the absence of theappreciable presence of the inert gas in the gaseous mixture, thedesired surface coating of polyphenylene polymer is not accomplishedbecause of the inability to achieve the requisite degree of excitationwhile maintaining moderate coating conditions, e.g., temperature. It isrecommended that the inert gas within the coating zone as well as thearomatic compound withinthe coating zone be either intermittently orcontinuously replenished (e.g., by the continuous introduction of each).

The aromatic compound which undergoes polymerization in the coating zoneto form a polyphenylene polymer coating or film upon the surface of thecarbon fiber has from 1 to 4 six member carbon rings (i.e., substitutedor unsubstituted benzene rings). When the aromatic compound containsmore than one six member carbon ring, the rings present in the compoundoptionally may be fused (i.e., condensed). The nature of the atoms orgroups of atoms (i.e., functional groups) which are additionally bondedto the carbon atoms of the six member carbon ring is not critical to theoperation of the present process. Other atoms or groups of atoms mayoptionally be substituted for the hydrogen atoms normally bonded to thering carbon atoms of such aromatic hydrocarbons. The presence offunctional groups bonded to the six member carbon ring of the aromaticcompound may, however, serve as a means for introducing functionalgroups into the resulting polyphenylene polymer which may ultimatelyserve to further enhance the adhesive bond between a carbon fiber and aspecific resin matrix material.

Illustrative examples of unsubstituted aromatic compounds are benzene,naphthalene, anthracene, phenanthrene, pyrene, and chrysene.

Illustrative examples of alkyl substituted aromatic compounds aretoluene, o-xylene, m-xylene, p-xylene, ethylbenzene, n-propylbenzene,cumene, nbutylbenzene, isobutylbenzene, p-ethyltoluene, diphenylmethane,l,2-diphenylethane, mesitylene, pentamethylbenzene, andhexamethylbenzene.

Illustrative examples of halogen substituted aromatic compounds arechlorobenzne, bormobenzene, fluorobenzene, o-dichlorobenzene,m-dichlorobenzene, pdichlorobenzene, l-bromo-4-chlorobenzene,pdibromobenzene, l ,2,3-trichlorobenzene, l ,3,5- trichlorobenzene,1,2,4-trichlorobenzene, hexafluorobenzene, hexachlorobenzene, and lchloronaphthalene.

Illustrative examples of additional monosubstituted aromatic compoundsinclude diphenyl, nitrobenzene, aniline, phenol, styrene,divinylbenzene, benzaldehyde, benzyl acetate, and benzoic acid.

Illustrative examples of disubstituted aromatic compounds having mixedfunctional groups are 0- chlorotoluene, m-chlorotoluene,p-chlorotoluene, o-

bromotoluene, p-bromotoluene, m-nitrobenzoic acid, 2,4-dinitrobenzoicacid, and p-bromoaniline.

The preferred aromatic compounds are those which exist as a liquid atroom temperature and which readily undergo volatilization.

The aromatic compound may be introduced into the coating zone by any oneof a variety of techniques, and the techniques selected is not criticalto the formation of the desired polyphenylene polymer coating. If thearomatic compound is a volatile liquid, it may be introduced into thecoating zone as a gaseous stream (e.g., the inert gas may be passedthrough a vessel containing the liquid aromatic compound prior to itsintroduction into the coating zone). If desired, the temperature of theliquid aromatic compound may be elevated in order to increase itsvolatility. Alternatively, the aromatic compound may be introduced intothe coating zone while present upon the carbonaceous fibrous material.For instance, the carbonaceous fibrous material may be initiallyimmersed in a liquid aromatic compound whereby the carbonaceous fibrousmaterial is impregnated with the same. Those aromatic compounds whichnormally exist as solids are conveniently dissolved in a solvent for thesame and the carbonaceous fibrous material immersed in the solutionprior to its introduction into the coating zone. When a solvent isemployed, the solvent may optionally be volatized prior to introductionof the impregnated carbonaceous fibrous material into the coating zone.If an aromatic solvent is introduced into the coating zone, the solventitself may also undergo the phenylene polymer formation reaction. Thedesired phenylene polymer formation reaction can be carried out (asdescribed hereafter) regardless of whether the aromatic compound isintroduced into the coating zone as a gas, a liquid, or as a solid. Whenthe aromatic compound is introduced as a liquid or as a solid, itpreferably is introduced while adhering to the carbonaceous fibrousmaterial.

The Formation of the Polyphenylene Polymer Coating The coating of thesurface of the carbonaceous fibrous material is accomplished bycontacting the carbonaceous fibrous material while present in thecoating zone with an excited gas species formed through the applicationof pulsed high frequency electrical power to the inert gas in thepresence of the aromatic compound. The carbonaceous fibrous material maybe statically suspended or otherwise positioned within the coating zone.In a preferred embodiment of the process a continuous length of thecarbonaceous fibrous material is continuously passed, e.g., in thedirection of its length, through the excited gas species present in thecoating zone. For instance, a rotating feed roll may be provided at theentrance end of the coating zone, and a rotating take-up roll may beprovided at the exit end of the coating zone.

The coating zone may be bounded by walls constructed of either aconductive or a non-conductive material. For instance, a tubular chamberconstructed of transplant glass may be conveniently selected to definethe bounds of the coating zone. In such an arrangement a continuouslength of carbonaceous fibrous material may be axially suspended thereinwith free access of its surface to the excited gas species provided.

The excited gas species required to produce the desired polyphenylenepolymer coating may be formed by inductively or capacitively couplingpulsed high frequency electric power to the contents of the coatingzone. A combination of inductive and capacitive coupling may also beutilized. As shown in FIG. 1 (described in detail hereafter), thecontents of the coating zone may be capacitively excited. Representativealternative apparatus arrangements wherein capacitive coupling also maybe utilized are shown in FIG. 1A, FIG. 1B, and FIG. 2. In FIG. 1A thepulsed high frequency electrical power is applied to metallic ringswhich are oriented perpendicularly to the axis of an elongated coatingzone and effectively surround the same. In FIG. 1B the pulsed highfrequency electrical power is applied to a pair of mercury filled tubesoriented parallel to the axis of an elongated coating zone andpositioned within the same. In FIG. 1C pulsed high frequency electricalpower is inductively applied to an elongated coating zone through theuse of a single coil which completely surrounds the same.

The term pulsed electrical power or electrical power in pulsed form asused herein is defined as pulses or bursts of high frequency electricalenergy, e.g., pulsed rf energy. The power may be an ac. signal having anamplitude of about 500 v. to 10 Kv. peak-topeak and a frequency of about0.5 KI-Iz, to 2,500 MHz. (preferably 1.0 KHz. to 30 MI-Iz.). The pulsesmay be from about 0.1 microseconds to 10 milliseconds duration(preferably 10 to 1,000 microseconds). The pulse repetition rate may befrom about 0.1 KI-Iz. to 20 MHz. (preferably about 1.0 to KHz). Thepulsed electrical power may be provided in accordance with techniquesknown to those skilled in the electrical arts, e. g., by gating a highfrequency oscillator or klystron on and off to generate bursts of highfrequency energy. The exact dimensions of the coating zone willinfluence the power requirement as will be apparent to those skilled inthe art.

The high frequency electrical power in pulsed form is applied to theinert gas in the presence of the aromatic compound in sufficientquantity to establish an excited gas species capable of forming apolyphenylene polymer coating while maintaining the temperature of thecoating zone at about 0 to C, and preferably at about 20 to 100C, andmost preferably at about 20 to 50C. When maintaining the temperture ofthe coating zone at the moderate temperatures indicated, a compactpolyphenylene polymer coating is formed similar in appearance to thatshown in FIG. 4. If the temperature of the coating zone is elevated muchabove about 150C., then a non-compact loosely adhering polyphenylenepolymer coating results which is similar in appearance to that shown inFIG. 5. If desired, the maintenance of the desired temperature may beaided by immersion of the coating zone in a low dielectric liquid bath,such as silicon oil.

The carbonaceous fibrous material is contacted with the excited gasspecies present within the coating zone until a compact coating of apolyphenylene polymer having a thickness of about 25 to-800 angstromunits (preferably 50 to 250 angstrom units) is deposited thereon and itsability to bond to a matrix material is beneficially enhanced. Unlikemany prior art surface modification techniques, the residence timerequired in the present process is relatively brief. For instance,residence times of about 0.2 to 20 minutes may be conveniently selected,and preferably residence times of about 1 to 4 minutes.

The surface modification process of the present invention offers theadvantage of uniformly altering the surface characteristics of thecarbonaceous fibrous materials to the substantial exclusion of adverselyinfluencing the single filament tensile properties of the same, i.e.,the tensile strength and Youngs modulus.

The surface modification of the present process makes possible improvedadhesive bonding between the carbonaceous fibers, and a resinous matrixmaterial. Accordingly, carbon fiber reinforced composite materials whichincorporate fibers coated as heretofore described exhibit enhanced shearstrength, fiexural strength, compressive strength, etc. The resinousmatrix material employed in the formation of such composite material iscommonly a polar thermosetting resin such as an epoxy, a polyimide, apolyester, phenolic, etc. The carbonaceous fibrous material is commonlyprovided in such resulting composite materials in ether an aligned orrandom fashion in a concentration of about 20 to 70 per cent by volume.

A representative apparatus arrangement for carrying out the surfacemodification process (i.e., coating process) of the invention isillustrated in FIG. 1. With reference to FIG. 1, the power unit includesa conventional variable dc. power supply 2, a conventional pulsegenerator 4 having a variable pulse repetition rate and a variable pulsewidth, a conventional signal amplifier 6, and a variable frequencyoscillator 8. The output signal from the pulse generator 4 is applied tothe oscillator 8 by way of the signal amplifier 6. Both a variablepositive dc. voltage and a fixed negative bias voltage from the powersupply 2 are applied to the oscillator 8.

The power supply 2 may be any conventional variable d.c. power supply,e.g., a Kepco Model 615B, -600 volt and negative 150 volt power supply.The pulse generator 4 may be any conventional pulse generaor of variablepulse repetition rate, e.g., a Hewlett Packard Model 3300A pulsegenerator, which provides pulses having a variable pulse repetition rateand either a constant or a selectably variable pulse width or duration.The amplifier 6 may be any conventional amplifier having an odd numberof stages which amplifies and inverts the pulses from the pulsegenerator 4 and provides positive output pulses. The oscillator 6 may beany conventional variable high frequency oscillator which preferablygenerates an output signal in the radio frequency range above 1.0KI-Iz., and which is capable of being gated or pulsed on and off toprovide bursts of high frequency energy. In a preferred operation of thepower unit this is accomplished by cutting off the oscillator byapplying a negative 150 volt bias to the control grid of an oscillatortube (not shown) by way of an input terminal 110 and by periodicallyapplying v positive pulses to the input terminal and thus the controlgrid of sufficient amplitude to drive the oscillator tube intoconduction.

In operation, the pulse generator 4 generates a series of negative goingpulses, the pulse repetition rate and- /or the pulse width of which maybe varied to thereby vary the reoccurrence rate and/or the duration ofthe pulses. The signal from the pulse generator 4 is amplified andinverted by the amplifier 6 and the positive pulses from the amplifier 6are applied to the oscillator 8. In the absence of a pulse from theamplifier 6, the oscillator 8 is cut off and does not provide an outputsignal. However, when a pulse from the pulse generator d is applied tothe oscillator 8 by way of the amplifier 6, the oscillator 8 breaks intohigh frequency oscillations and provides an output signal for theduration of the applied pulse. The resultant pulsed high frequencysignal may be coupled to the coating zone 20 through a conventional highfrequency step-up coil 12, the primary winding of which may be utilizedfor both signal coupling and as a portion of the oscillator tankcircuit. Lead 14 connects the coil 12 to coaxial electrode 21. Coaxialelectrode 21 consists of a 10 inch length of copper tubing having anouter diameter of one-half inch and an inner diameter of seven-sixteenthinch. Situated in series with coaxial electrode 211 is a like coaxialelectrode 22.

The amplitude of the output signal from the oscillator 6 may be variedby varying the voltage directly applied to the oscillator 8 from thepower supply 2. The frequency of the output signal from the oscillator 8may, of course, be varied in any suitable conventional manner, e.g., byvarying the reactive avlue of an electn'cal component in a tank circuit(not shown). In addition, the relationship between the on" time and theoff time of the output signal and the duration of the pulses of highfrequency energy may be varied by adjusting the pulse repetition rateand/or width of the output pulses from the pulse generator 4. The pulseunit is thus capable of supplying bursts of electrical energy of avariable high frequency, the bursts occurring at a selectable burstrepetition rate and having a variable burst width or duration.

Another representative pulsing unit which may be used to provide thepulsed high frequency signal to excite the inert gas in the coating zoneis a Lepel Model No. T-53 high frequency power unit capable ofdelivering up to a 10 Kv. signal at a frequency of up to 30 MHz. pulsedby a grid pulse modulator Model 1414 available from Pulse TronicsEngineering Co.

By providing a pulsed frequency signal as described above, excessiveheat buildup within the coating zone 20 may be prevented throughvariation of the pulse repetition rate, the pulse width or duration, orboth of these parameters. The heat generated within the coating zoneduring the application of pulsed high frequency signal is allowed todissipate to a great extent during the off period of the oscillator,i.e., between pulses of high frequency energy.

Since the signal amplitude, frequency, duration and repetition raterequired for carrying out the process depend upon the diameter andlengthof the coating zone, such parameters may vary widely. The temperatureinside the coating zone 20 may be sensed by a thermocouple 23 and avisual temperature indication may be provided at meter 25. Thetemperature within the zone 20 may thus be easily regulated by visuallymonitoring the meter 25 and adjusting the pulse repetition rate and/orthe pulse width of the high. frequency signal. The intensity of theexcitation is controlled by the amplitude and duration of the pulses,the pulse repetition rate, the space gap between the electrodes, and thetotal length of the coating zone.

With a coating zone or chamber 20 of approximately 22 inches in lengthand seven-sixteenth inch in diameter, the process may be convenientlypracticed utilizing a pulsed high frequency output signal from theoscillator 6 in the radio frequency range above 1.0 KHz, theparticularly preferred range being from 1.0 Kl-Iz. to 30 MHZ. The signalmay be pulsed at a repetition rate of Ill from about 1.0 to about 1,000KHz. to 100 KHZ. being preferred) while the pulse width may be from 0.1to 1,000 microseconds, (1.0 to 500 microseconds being preferred). Theamplitude of the pulsed high frequency signal may be from 500 v. to 10Kv. (1 to 5 Kv. being preferred).

The following examples are given as specific illustrations of theprocess of the invention. It should be understood, however, that theinvention is not limited to the specific details set forth in theexamples. EXAM- PLE I Reference is made to the apparatus of FIG. 1.

A high strength-high modulus continuous filament carbonaceous yarnderived from an acrylonitrile homopolymer in accordance with theprocedures described in United States Ser. Nos. 749,957, filed Aug. 5,1968, and 777,957, filed Nov. 20, 1968 (now abandoned) is selected asthe starting material. The yarn consists of a 1,600 fil bundle having atotal denier of about 1,000, has a carbon content in excess of 99 percent by weight, exhibits a predominantly graphite x-ray diffractionpattern, a single filament tenacity of about 13 grams per denier and asingle filament Youngs modulus of about 50 million psi.

The carbonaceous yarn 24 is unwound from rotating feed roll 26 and isimmersed in benzene 60 which is provided in vessel 62 by the aid ofrollers 64, 66, and 68. The resulting benzene impregnated carbonaceousfibrous material next passes through neck 27, around pulley 28, throughcoating zone 20, through neck 32, around pulley 34, and is ultimatelytaken up upon rotating uptake roll 36. The carbonaceous yarn 24 passesthrough coating zone while axially suspended therein at a rate of 12inches per minute.

The coating zone 20 is defined by tubular glass of about seven-sixteenthinch diameter and about 22 inches in length. Helium is introduced as theinert gas via inlet tube at a rate of 2,000 cc. per minute. Off

gases exit via exit tube 42. A 3,000 v. peak-to-peak a.c. signal havinga frequency of 13.56 MHz. is applied to coaxial electrode 211 in pulsesof 500 microseconds duration of p.r.r. (pulse repetition rate) of 100KHz. An excited gas species is established throughout the length of thecoating zone 20. The yarn 24 is in contact with the excited gas speciesfor a residence time of about 2 minutes and a compact polyphenylenepolymer coating is formed thereon having a thickness of about 100angstrom units. Throughout the phenylene polymer coating treatment thetemperature within zone 20 is maintained at approximately 25C. asmeasured by thermocouple 23 and indicated on meter 25. During thecoating deposition the entire coating zone 20 is surrounded by coolingbath 46 of silicone oil, which is kept in circulation by a pump 48connected to reservoir 50 via lines 52 and 54. The polyphenylene natureof the polymer coating is confirmed by infra red analysis. Thecarbonaceous yarn following surface treatment retains its originaltenacity and Youngs modulus.

A composite article is next formed employing the polyphenyl polymercoated yarn sample as a reinforcing medium in a resinous matrix. Thecomposite article is a rectangular bar consisting of about 65 per centby volume of the yarn and having dimensions of A: inch X V4 inch X5inches. The composite article is formed by impregnation of the coatedyarn in a liquid epoxy resinhardener mixture at 50C. followed byunidirectional layup of the required quantity of the impregnated yarn ina steel mold and compression molding of the layup for 2 hours at 93C.,and 2.5 hours at 200C. in a heated platen press at about 100 psipressure. The mold is cooled slowly to room temperature, and thecomposite article is removed from the mold cavity and cut to size fortesting. The resinous matrix material used in the formation of thecomposite article is provided as a solventless system which contains 100parts by weight epoxy resin and 88 parts by weight of anhydride curingagent.

The horizontal interlaminar shear strength is determined by short beamtesting of the carbon fiber reinforced composite according to theprocedure of ASTM D2344-65T as modified for straight bar testing at a4:1 span to depth ratio and found to be substantially greater than thatof a control wherein an identical carbonaceous yarn serving as thefibrous reinforcement is never subjected to any form of surfacemodification.

EXAMPLE II Example I is repeated with the exception that the aromaticcompound is toluene.

Substantially similar results are achieved.

EXAMPLE III Example I is repeated with the exception that the aromaticcompound 60 is styrene.

Substantially similar results are achieved.

EXAMPLE IV Example I is repeated with the exception that the aromaticcompound 60 is aniline.

Substantially similar results are achieved.

EXAMPLE V Example I is repeated with the exception that a mixture ofaromatic compounds 60 is provided in vessel 62. The mixture of aromaticcompounds consists of 10 per cent by weight solution of phenol dissolvedin a benzene solvent.

EXAMPLE VI having a total denier of about 700.

When incorporated in a composite article (as described), the surfacecoated yam produces a composite article exhibiting an enhancedhorizontal interlaminar shear strength when compared with that of acontrol wherein the yarn underwent no form of surface modification.

EXAMPLE VII Reference is made to the apparatus of FIG. 2 in which likenumerals designate similar components to those previously described inconnection with FIG. 1.

A carbonaceous multifilament yarn 24 identical to that employed inExample I is continuously unwound from feed roll 29 and passed throughmercury seal 31 supplied from reservoir 33.

The carbonaceous yarn is passed through coating zone 20 at a rate of 16inches per minute, and is axially suspended within the center of thezone by the aid of annular guide 30 and neck 32. Helium is introduced asthe inert gas via inlet tube 70 at a rate of 2,000 0.0. per minute. Aportion of the helium passes through conduits 72 and 80 at a rate of1,600 c.c./min. as controlled by valve 7'6. The remaining portion of thehelium passes through conduit 7d via valve 78 at a rate of 400 c.c./min.and is bubbled through benzene 84 provided at room temperature (i.e.,about 25C.) in vessel 86. The helium together with volatilized benzeneis next introduced into the coating zone 20 via inlet tube 40.

The gaseous helium and benzene present in the coating zone 20 is excitedby means of the capacitance between mercury jacket l7 encircling theelectrically grounded carbonaceous yarn.

A Pulse Tronics generator is used to control a Lepel Model T-3 highfrequency signal generator to provide a 3,000 v. peak-to-peak a.c.signal at a frequency of 10 MHz. in pulses of 500 microseconds durationat a pm. of 10 KHz. An excited gas species is established throughout thelength of the coating zone 20. The yarn 24 is in contact with theexcited gas species for a residence time of about 1 minute during whichtime a compact coating of a polyphenylene polymer having a thickness ofabout 100 angstrom units is uniformly deposited thereon. Throughout thecoating treatment the temperature within zone 20 is maintained atapproximately 25 c. as measured by thermocouple 23 and indicated onmeter 25.

Substantially similar surface modifications results are achieved.

The nature, scope, utility, and effectiveness of the present inventionhave been described and specifically exemplified in the foregoingspecification. However, it should be understood that these examples arenot in tended to be limiting and that the scope of the invention to beprotected is particularly pointed out in the appended claims.

I claim:

1.. A composite article exhibiting enhanced interlaminar shear strengthcomprising a resinous matrix material derived from a thermosetting resinhaving incorporated therein a carbonaceous fibrous, material containingat least per cent carbon by weight which bears upon the surface of saidcarbonaceous fibrous material a compact coating of polyphenylene polymerhaving a thickness of about 25 to 800 angstrom units.

2. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim ll wherein said resinous matrix material isderived from a polar thermosetting resin selected from the groupconsisting essentially of an epoxy, a polyimide, a polyester, and aphenolic resin.

3. A composite article exhibiting; enhanced interlaminar shear strengthin accordance with claim ll wherein said carbonaceous fibrous materialcontains at least per cent carbon by weight.

4. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim 1 wherein said carbonaceous fibrous materialincludes graphitic carbon.

5. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim ll wherein said compact coating of apolyphenylene polymer is de rived from benzene.

6. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim 1 wherein said compact coating of apolyphenylene polymer is derived from toluene.

7. A composed article exhibiting enhanced interlaminar shear strength inaccordance with claim 1 wherein said compact coating of a polyphenylenepolymer is derived from styrene.

8. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim It wherein said compact coating ofpolyphenylene polymer is de rived from aniline.

9. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim 1 wherein said compact coating of polyphenylenepolymer has a thickness of about 50 to 250 angstrom units.

i= =l =i= =i

1. A COMPOSITE ARTICLE EXHIBITING ENHANCED INTERLAMINAR SHEAR STRENGTHCOMPRISING A RESINOUS MATRIX MATERIAL DERIVED FROM A THERMOSETTING RESINHAVING INCORPORATED THEREIN A CARBONACEOUS FIBROUS MATERIAL CONTAININGAT LEAST 90 PER CENT CARBON BY WEIGHT WHICH BEARS UPON THE SURFACE OFSAID CARBO-
 2. A composite article exhibiting enhanced interlaminarshear strength in accordance with claim 1 wherein said resinous matrixmaterial is derived from a polar thermosetting resin selected from thegroup consisting essentially of an epoxy, a polyimide, a polyester, anda phenolic resin.
 3. A composite article exhibiting enhancedinterlaminar shear strength in accordance with claim 1 wherein saidcarbonaceous fibrous material contains at least 95 per cent carbon byweight.
 4. A composite article exhibiting enhanced interlaminar shearstrength in accordance with claim 1 wherein said carbonaceous fibrousmaterial includes graphitic carbon.
 5. A composite article exhibitingenhanced interlaminar shear strength in accordance with claim 1 whereinsaid compact coating of a polyphenylene polymer is derived from benzene.6. A composite article exhibiting enhanced interlaminar shear strengthin accordance with claim 1 wherein said compact coating of apolyphenylene polymer is derived from toluene.
 7. A composed articleexhibiting enhanced interlaminar shear strength in accordance with claim1 wherein said compact coating of a polyphenylene polymer is derivedfrom styrene.
 8. A composite article exhibiting enhanced interlaminarshear strength in accordance with claim 1 wherein said compact coatingof polyphenylene polymer is derived from aniline.
 9. A composite articleexhibiting enhanced interlaminar shear strength in accordance with claim1 wherein said compact coating of polyphenylene polymer has a thicknessof about 50 to 250 angstrom units.